This invention relates to optical distance measurement apparatus for determining the distance to an object on the basis of the round-trip time of the light radiated from the apparatus and reflected by the object, and more particularly to the smear detector device for detecting the smear upon the windows in the light emitting and receiving path of the optical distance measurement apparatus.
FIG. 9 is a block diagram showing a conventional smear detector device for an optical distance measurement apparatus, which is disclosed, for example, in Japanese Patent Publication (Kokoku) No. 3-30116. The light emitted from the light emitter unit 1 is radiated on an object 7. The light emitter unit 1 includes a laser diode 11, a pulse driver circuit 12 and a emitter lens 13. The laser diode 11 is driven by the pulse driver circuit 12 to generate pulses of light, which is converged by the emitter lens 13 toward the object 7. The light reflected from the object 7 is received by a light receiver unit 2. The received light is converged by a receiver lens 21 upon a light receiving element 22. A processing circuit 3 calculates the distance to the object 7 based on the measurement of the interval of time between the excitation of the laser diode 11 by means of the pulse driver circuit 12 and the detection of the reflected light by means of the light receiving element 22. Transparent glass plates are positioned in front of the light emitter unit 1 and the light receiver unit 2, respectively, to form a emitter window 4 and a receiver window 5.
A smear detector unit 6 detects the blur or smear attached, for example, upon the emitter window 4. A light receiver element 61 receives the light scattered by the emitter window 4 and guided through a shield 63, and generates an electrical signal corresponding to the intensity of the received light. A smear detection processing circuit 62 detects the smear on the emitter window 4 on the basis of the level of the electrical signal that is output from the light receiver element 61.
Next, the operation of the apparatus of FIG. 9 is described. When excited by the pulse driver circuit 12, the laser diode 11 outputs a pulsed laser light. The laser light is converged by the processing circuit 3 and emitted forward through the emitter window 4. If an object 7 exists in front of the apparatus, the light reflected by the object 7 is received by the light receiver unit 2 through the receiver window 5. The received light is converged by the receiver lens 21 upon the light receiving element 22, which effects the photoelectric conversion to obtain an electrical signal corresponding to the intensity of the received light.
The processing circuit 3 compares the signal output from the light receiver unit 2 with a threshold level, and, if the signal level is greater than the threshold level, the processing circuit 3 judges that the signal corresponds to the light reflected from the object 7. Further, using a high-speed counter (not shown), the processing circuit 3 measures the time interval between the excitation of the emitter lens 13 by the pulse driver circuit 12 and the reception of the light at the light receiving element 22. The measurement corresponds to the round-trip time in which the laser light goes forward from the apparatus to the object 7 and thence backward to the apparatus. Finally, the processing circuit 3 determines the distance between the apparatus and the object 7 from the round-trip time, using the following formula: EQU (DISTANCE)=(ROUND-TRIP TIME).multidot.(LIGHT SPEED)/2 (1)
The smear detector unit 6, on the other hand, receives the light scattered at the inner and outer interfaces of the emitter window 4, and outputs an electrical signal corresponding to the intensity of the scattered light. The intensity of the scattered light corresponds to the amount of smear attached on the emitter window 4. Thus, the smear detection processing circuit 62 judges that the emitter window 4 is not smeared or blurred if the output of the light receiver element 61 is below a predetermined level. If, on the other hand, the output of the light receiver element 61 exceeds the predetermined level, the smear detection processing circuit 62 judges that the emitter window 4 is smeared and outputs a smear detection signal.
The smear detector device of FIG. 9, however, has the following disadvantage. Namely, the light scattered by the smear upon the emitter window 4, etc., is guided to the light receiver element 61 via the shield 63. The smear is detected on the basis of the intensity of the scattered light received on the light receiver element 61. Thus, the smear detection area is limited by the shield 63 within the region of the emitter window 4 on which the light is radiated. The smear detection may thus become unreliable as described next.
If the smear is spread uniformly over the emitter window 4, the level of the scattered light corresponds to the amount of smear which interferes with the distance detection. If, however, the smear is concentrated to a region outside of the smear detection area, the level of the scattered light detected by the light receiver element 61 remains low although a significant part of the light radiated from the light emitter unit 1 is scattered by the emitter window 4. The sensitivity of the smear detection thus deteriorates. On the other hand, if the smear is concentrated within the smear detection area, the level of the scattered light detected by the light receiver element 61 becomes higher than level corresponding to the overall amount of the smear attached upon the emitter window 4. The smear detector device thus becomes hyper-sensitive.
Further disadvantage of the smear detector device of FIG. 9 is that the smear detector device should be provided with a separate light receiver element 61 and a separate smear detection processing circuit 62. The device is thus expensive.